coverage: Tweak individual tests to be unaffected by rustfmt

Some of these tests use non-standard formatting that we can simulate by
strategically adding `//` line comments.

One contains `where` clauses that would be split across multiple lines, which
we can keep on one line by moving the bounds to the generic type instead.

tests/coverage/while.rs

@@ -1,5 +1,6 @@
 fn main() {
     let num = 9;
     while num >= 10 {
+        // loop body
     }
 }